Method of producing high-strength hot-dip galvanized steel sheet and method of producing high-strength galvannealed steel sheet

ABSTRACT

A high-strength hot-dip galvanized steel sheet is produced by a method that includes a first heating step of holding a steel sheet at a temperature for a time in an atmosphere having a H 2  concentration and a dew point, a cooling step of cooling the steel sheet after the first heating step, an electrolytic treatment step of subjecting the steel sheet after the cooling step to electrolytic treatment under specific conditions, a second heating step of holding the steel sheet after the electrolytic treatment step at a temperature for a time in an atmosphere having a H 2  concentration and a dew point, and a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.

TECHNICAL FIELD

This disclosure relates to a method of producing a high-strength hot-dip galvanized steel sheet suitable for automotive body components and has a good surface appearance and good adhesion of the coating and to a method of producing a high-strength galvannealed steel sheet.

BACKGROUND

In recent years, with the increase in global environmental protection awareness, there has been a strong demand for improved mileage to reduce CO₂ emissions from automobiles. To satisfy the demand, a strong movement is under way to strengthen steel sheets used as automotive body component materials to decrease the thickness of automotive body components and thereby decrease the weight of automotive bodies. However, strengthened steel sheets may have low ductility. Thus, it is desirable to develop high-strength high-ductility steel sheets.

Solid-solution strengthening elements such as Si, Mn, and/or Cr have been added to strengthen steel sheets. In particular, Cr in a smaller amount than other elements can strengthen steel sheets. Thus, the addition of Cr is effective in strengthening the material property of steel sheets. However, Cr and such elements are more oxidizable elements than Fe. Thus, there are problems as described below in the production of hot-dip galvanized steel sheets and galvannealed steel sheets based on high-strength steel sheets containing large amounts of such elements.

In general, to produce a hot-dip galvanized steel sheet, a steel sheet is heated and then annealed in a nonoxidizing atmosphere or a reducing atmosphere at a temperature of approximately 600° C. to 900° C. and then subjected to hot-dip galvanizing treatment. Oxidizable elements in steel are selectively oxidized even in a nonoxidizing atmosphere or reducing atmosphere generally employed, are concentrated on the surface, and form oxides on the surface of the steel sheet. Such oxides lower the wettability of molten zinc on the surface of the steel sheet in hot-dip galvanizing treatment and cause an ungalvanized surface. An increase in the concentration of oxidizable elements in steel drastically lowers the wettability and frequently causes ungalvanized surfaces. Even if ungalvanized surfaces are not formed, oxides existing between the steel sheet and the coating reduce adhesion of the coating.

To address this issue, a method of improving the wettability of molten zinc on a surface of a steel sheet is disclosed in Japanese Patent No. 2587724. That method includes heating a steel sheet in an oxidizing atmosphere in advance to rapidly form an iron oxide film on a surface of the steel sheet at an oxidation rate equal to or higher than a predetermined value, thereby preventing oxidation of additive elements on the surface of the steel sheet, and thereafter subjecting the iron oxide film to reduction annealing. However, if a steel sheet is excessively oxidized, iron oxide adheres to a hearth roll and causes a problem of indentation flaws on the steel sheet.

A method disclosed in Japanese Patent No. 3956550 includes pickling a steel sheet after annealing to remove oxides from the surface and thereafter annealing again and hot-dip galvanizing the steel sheet. However, oxides that are insoluble in acids cannot be removed in Japanese Patent No. 3956550. Thus, the appearance of the coating of steel sheets containing Cr that forms an oxide insoluble in acids cannot be improved.

A method disclosed in Japanese Unexamined Patent Application Publication No. 2001-158918 includes immersing a steel sheet in an alkaline molten salt bath after annealing to remove silicon-based oxides, then annealing the steel sheet again, and hot-dip galvanizing the steel sheet. However, electrolytic treatment is not performed in Japanese Unexamined Patent Application Publication No. 2001-158918. Thus, chromium oxide cannot be removed and the appearance of the coating of steel sheets containing Cr cannot be improved.

In view of such situations, it could be helpful to provide a method of producing a high-strength hot-dip galvanized steel sheet having good adhesion of the coating and a good surface appearance and a method of producing a high-strength galvannealed steel sheet.

SUMMARY

We thus provide:

-   -   (1) A method of producing a high-strength hot-dip galvanized         steel sheet, including a first heating step of holding a steel         sheet at a temperature in the range of 700° C. to 900° C. or in         a temperature range of 700° C. to 900° C. for 20 to 600 seconds         in an atmosphere having a H₂ concentration in the range of 0.05%         to 25.0% by volume and a dew point in the range of −45° C. to 0°         C., the steel sheet having a composition of C: 0.040% to 0.500%,         Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 5.00% or less, P:         0.100% or less, S: 0.0100% or less, and Al: 0.100% or less on a         mass percent basis, the remainder being Fe and incidental         impurities; an electrolytic treatment step of subjecting the         steel sheet after the first heating step to electrolytic         treatment in an alkaline aqueous solution at a charge density in         the range of 1.0 to 400 C/dm², the steel sheet acting as an         anode; a second heating step of holding the steel sheet after         the electrolytic treatment step at a temperature in the range of         650° C. to 900° C. or in a temperature range of 650° C. to         900° C. for 15 to 300 seconds in an atmosphere having a H₂         concentration in the range of 0.05% to 25.0% by volume and a dew         point of 0° C. or less; and a coating treatment step of         subjecting the steel sheet after the second heating step to         hot-dip galvanizing treatment.     -   (2) A method of producing a high-strength hot-dip galvanized         steel sheet, including a first heating step of holding a steel         sheet at a temperature in the range of 700° C. to 900° C. or in         a temperature range of 700° C. to 900° C. for 20 to 600 seconds         in an atmosphere having a H₂ concentration in the range of 0.05%         to 25.0% by volume and a dew point in the range of −45° C. to 0°         C., the steel sheet having a composition of C: 0.040% to 0.500%,         Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 8.00% or less, P:         0.100% or less, S: 0.0100% or less, and Al: 0.100% or less on a         mass percent basis, the remainder being Fe and incidental         impurities; an electrolytic treatment step of subjecting the         steel sheet after the first heating step to electrolytic         treatment in an alkaline aqueous solution at a charge density in         the range of 1.0 to 400 C/dm², the steel sheet acting as an         anode; a pickling step of pickling the steel sheet after the         electrolytic treatment such that a pickling weight loss ranges         from 0.05 to 5 g/m² on an Fe basis; a second heating step of         holding the steel sheet after the pickling step at a temperature         in the range of 650° C. to 900° C. or in a temperature range of         650° C. to 900° C. for 15 to 300 seconds in an atmosphere having         a H₂ concentration in the range of 0.05% to 25.0% by volume and         a dew point of 0° C. or less; and a coating treatment step of         subjecting the steel sheet after the second heating step to         hot-dip galvanizing treatment.     -   (3) A method of producing a high-strength hot-dip galvanized         steel sheet, including a first heating step of holding a steel         sheet at a temperature in the range of 700° C. to 900° C. or in         a temperature range of 700° C. to 900° C. for 20 to 600 seconds         in an atmosphere having a H₂ concentration in the range of 0.05%         to 25.0% by volume and a dew point in the range of −45° C. to 0°         C., the steel sheet having a composition of C: 0.040% to 0.500%,         Si: 1.00% or less, Cr: 0.10% to 3.00%, Mn: 8.00% or less, P:         0.100% or less, S: 0.0100% or less, and Al: 0.100% or less on a         mass percent basis, the remainder being Fe and incidental         impurities; a pickling step of pickling the steel sheet after         the first heating step such that a pickling weight loss ranges         from 0.05 to 5 g/m² on an Fe basis; an electrolytic treatment         step of subjecting the steel sheet after the pickling step to         electrolytic treatment in an alkaline aqueous solution at a         charge density in the range of 1.0 to 400 C/dm², the steel sheet         acting as an anode; a second heating step of holding the steel         sheet after the electrolytic treatment step at a temperature in         the range of 650° C. to 900° C. or in a temperature range of         650° C. to 900° C. for 15 to 300 seconds in an atmosphere having         a H₂ concentration in the range of 0.05% to 25.0% by volume and         a dew point of 0° C. or less; and a coating treatment step of         subjecting the steel sheet after the second heating step to         hot-dip galvanizing treatment.     -   (4) The method of producing a high-strength hot-dip galvanized         steel sheet according to (1) to (3), wherein the electrolytic         treatment in the alkaline aqueous solution in the electrolytic         treatment step is performed for 2 seconds or more.     -   (5) The method of producing a high-strength hot-dip galvanized         steel sheet according to (1) to (4), wherein the composition         further includes at least one element selected from Mo: 0.01% to         0.50%, Nb: 0.010% to 0.100%, B: 0.0001% to 0.0050%, and Ti:         0.010% to 0.100% on a mass percent basis.     -   (6) The method of producing a high-strength hot-dip galvanized         steel sheet according to any one of (1) to (5), wherein the         composition further includes at least one element selected from         Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N:         0.0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca:         0.0100% or less, and REM: 0.010% or less on a mass percent         basis.     -   (7) A method of producing a high-strength galvannealed steel         sheet, including subjecting a high-strength hot-dip galvanized         steel sheet to alloying treatment, wherein the high-strength         hot-dip galvanized steel sheet is produced by the method         according to any one of (1) to (6).

We provide a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet each having high strength, a good surface appearance, and good adhesion of the coating. For example, application of a high-strength hot-dip galvanized steel sheet to automobile structural members can improve mileage due to weight reduction of automotive bodies.

DETAILED DESCRIPTION

Annealing a steel sheet forms chromium oxide on the surface of the steel sheet and forms a low Cr concentration region in a surface layer of the steel sheet. If chromium oxide on the surface can be removed, because of the low concentration of Cr in the surface layer of the steel sheet, another annealing forms less chromium oxide, that is, a suppressed amount of chromium oxides on the surface of the steel sheet. However, chromium oxides are insoluble in acids, as described above. Thus, to solve problems caused by chromium oxide by a method of removing chromium oxide from the surface, chromium oxide on the surface must be removed by a method other than acid dissolution. The term a “surface layer” of steel sheets, as used herein, refers to a layer having a thickness of 10 μm or less directly under the surface of the steel sheets.

In an electric potential-pH diagram of Cr, chromic acid is stable in a wide acidic to alkaline region on a high electric potential side. That is, chromium oxide can be converted into chromic acid in an aqueous solution by application of a high electric potential to be dissolved in the aqueous solution. Chromic acid is formed at a lower electric potential in an alkaline region than in an acidic region. Thus, we extensively studied the assumption that the appearance of the coating of steel sheets containing Cr can be improved by removing chromium oxide by converting it into chromic acid dissolved in an alkaline aqueous solution. We found that chromium oxide can be removed by electrolytic treatment using a steel sheet as an anode in an alkaline aqueous solution. Chromium oxide on a surface of a steel sheet can be removed even in a short electrolytic treatment time. Further, chromium oxide at grain boundaries in steel sheets can also be removed in an electrolytic treatment time of 2 seconds or more. This particularly improves the adhesion of the coating of hot-dip galvanized steel sheets.

Our methods and steel sheets will be described below. This disclosure, however, is not limited to the examples. The symbol “%” that represents the amount of component refers to “% by mass”.

FIRST EXAMPLE

First, a steel sheet that is used as a raw material in a production method according to a first example will be described below. The steel sheet used as a raw material contains C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 5.00% or less, P: 0.100% or less,

S: 0.0100% or less, and Al: 0.100% or less on a mass percent basis, and the remainder is Fe and incidental impurities. In addition to these components, the steel sheet may contain at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0.0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. In addition to these components, the steel sheet may contain at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, N: 0.0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis. Each of the components listed above will be described below.

C: 0.040% to 0.500%

C is an austenite formation element, forms a composite microstructure in an annealed sheet, and is effective in improving strength and ductility. The C content is 0.040% or more to improve strength and ductility. However, a C content of more than 0.500% results in marked hardening of a welded portion and a heat-affected zone, degradation of the mechanical characteristics of the welded portion, poor spot weldability, and poor arc weldability. Thus, the C content is 0.500% or less.

Si: 1.00% or less

Si is a ferrite formation element and is effective in improving the solid-solution strengthening and work hardenability of ferrite in annealed sheets. A Si content of more than 1.00% results in formation of silicon oxide on a surface of a steel sheet during annealing and consequently poor coatability. Thus, the Si content is 1.00% or less. The Si content may be 0%.

Cr: 0.10% to 2.00%

Cr is an austenite formation element and effective in securing the strength of annealed sheets. The Cr content is 0.10% or more to secure the strength. However, a Cr content of more than 2.00% results in insufficient removal of chromium oxide from a surface layer of a steel sheet resulting in a poor appearance of the coating. Thus, the Cr content is 2.00% or less.

In the production method according to the first example, the remainder may be Fe and incidental impurities. As described above, the steel sheet used as a raw material in the production method according to the first example may contain the following components in addition to the components described above.

Mn: 5.00% or less

Mn is an austenite formation element and effective in securing the strength of annealed sheets. The Mn content is preferably 0.80% or more to secure the strength. However, a Mn content of more than 5.00% may result in a poor appearance of the coating due to a large amount of oxide formed in a surface layer of a steel sheet during annealing. Thus, the Mn content is preferably 5.00% or less.

P: 0.100% or less

P is an element effective in strengthening steel. However, a P content of more than 0.100% may result in embrittlement due to intergranular segregation and low impact resistance. Thus, the P content is preferably 0.100% or less.

S: 0.0100% or less

S forms an inclusion such as MnS, and thereby lowers impact resistance or causes a crack along a metal flow in a welded portion. Thus, the S content is preferably minimized. The S content is preferably 0.0100% or less.

Al: 0.100% or less

Excessive addition of Al results in low surface quality or poor formability due to an increased oxide inclusion and is responsible for increased costs. Thus, the Al content is preferably 0.100% or less, more preferably 0.050% or less.

Mo: 0.01% to 0.50%

Mo is an austenite formation element and effective in securing the strength of annealed sheets. The Mo content is preferably 0.01% or more to secure the strength. Owing to high alloy costs of Mo, a high Mo content may be responsible for increased costs. Thus, the Mo content is preferably 0.50% or less.

Nb: 0.010% to 0.100%

Nb is an element that contributes to improved strength due to solid-solution strengthening or precipitation strengthening. The Nb content is preferably 0.010% or more to produce this effect. However, a Nb content of more than 0.100% may result in low ductility and poor workability of steel sheets. Thus, the Nb content is preferably 0.100% or less.

B: 0.0001% to 0.0050%

B improves hardenability and contributes to improved strength of steel sheets. The B content is preferably 0.0001% or more to produce this effect. However, an excessively high B content may result in low ductility and poor workability. An excessively high B content is also responsible for increased costs. Thus, the B content is preferably 0.0050% or less.

Ti: 0.010% to 0.100%

Ti, together with C or N, forms fine carbide or fine nitride in steel sheets and contributes to improved strength of the steel sheets. The Ti content is preferably 0.010% or more to produce this effect. This effect levels off at a Ti content of more than 0.100%. Thus, the Ti content is preferably 0.100% or less.

Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less

Cu, V, and Ni are effective in strengthening steel and may be used to strengthen steel within the ranges specified herein. To strengthen steel, the Cu content is preferably 0.05% or more, the V content is preferably 0.005% or more, and the Ni content is preferably 0.05% or more. However, an excessively high Cu content of more than 1.00%, an excessively high V content of more than 0.500%, or an excessively high Ni content of more than 0.50% may result in low ductility due to markedly increased strength. An excessively high content with respect to these elements may be responsible for increased costs. Thus, when these elements are added, the Cu content is preferably 1.00% or less, the V content is preferably 0.500% or less, and the Ni content is preferably 0.50% or less.

N: 0.0100% or less

N reduces the anti-aging effects of steel and is preferably minimized. AN content of more than 0.0100% may result in significantly reduced anti-aging effects. Thus, the N content is preferably 0.0100% or less.

Sb: 0.10% or less, Sn: 0.10% or less

Sb and Sn can suppress nitriding in the vicinity of a surface of a steel sheet. To suppress nitriding, the Sb content is preferably 0.005% or more, and the Sn content is preferably 0.005% or more. This effect levels off at a Sb content or a Sn content of more than 0.10%. Thus, the Sb content is preferably 0.10% or less, and the Sn content is preferably 0.10% or less.

Ca: 0.0100% or less

Ca is effective in improving ductility due to the shape control of a sulfide such as MnS. The Ca content is preferably 0.001% or more to produce this effect. This effect levels off at a Ca content of more than 0.0100%. Thus, the Ca content is preferably 0.0100% or less.

REM: 0.010% or less

REM controls the shape of sulfide-base inclusions and contributes to improved workability. The REM content is preferably 0.001% or more to produce the effect of improving workability. A REM content of more than 0.010% may result in an increased amount of inclusions and poor workability. Thus, the REM content is preferably 0.010% or less.

The remainder is Fe and incidental impurities.

The method of producing the steel sheet used as a raw material in the production method according to the first example is not particularly limited. For example, a steel slab having the composition as described above is heated and then subjected to rough rolling and finish rolling in a hot-rolling step. Then, scales are removed from a surface layer of the hot-rolled steel sheet in a pickling step, and the hot-rolled steel sheet is subjected to cold rolling. The conditions for the hot-rolling step and the conditions for the cold-rolling step are not particularly limited and may be appropriately determined.

The steel sheet used as a raw material is typically produced through such common steps of steel making, casting, and hot rolling as described above. However, for example, part or all of the hot-rolling step may be omitted by using strip casting or the like.

The production method according to the first example will be described below. The production method includes a first heating step, a cooling step, an electrolytic treatment step, a second heating step, and a coating treatment step.

First Heating Step

The first heating step includes holding the steel sheet at a temperature of 700° C. to 900° C. or 700° C. to 900° C. for 20 to 600 seconds in an atmosphere having a H₂ concentration of 0.05% to 25.0% by volume and a dew point of −45° C. to 0° C. In the first heating step, while Fe is not oxidized, Cr is oxidized on the surface of the steel sheet. Consequently, a surface layer containing chromium oxide is formed.

The H₂ concentration should be high enough to suppress the oxidation of Fe and is 0.05% by volume or more. A H₂ concentration of more than 25.0% by volume results in increased costs. Thus, the H₂ concentration is 25.0% by volume or less.

A dew point of less than −45° C. results in suppressed oxidation of Cr. A dew point of more than 0° C. results in oxidation of Fe. Thus, the dew point ranges from −45° C. to 0° C.

A steel sheet temperature of less than 700° C. results in no oxidation of Cr. A steel sheet temperature of more than 900° C. results in high heating costs. Thus, the heating temperature of the steel sheet (steel sheet temperature) is a temperature of 700° C. to 900° C. Holding in the first heating step may be holding of the steel sheet at a constant temperature or may be holding of the steel sheet at varying temperatures.

A holding time of less than 20 seconds results in insufficient formation of chromium oxide on the surface. A holding time of more than 600 seconds causes low electrolytic treatment efficiency due to excessive formation of chromium oxide and results in low production efficiency. Thus, the holding time ranges from 20 to 600 seconds.

Electrolytic Treatment Step

The electrolytic treatment step includes subjecting the steel sheet after the first heating step to electrolytic treatment in an alkaline aqueous solution at a Coulomb density (charge density) of 1.0 to 400 C/dm², where the steel sheet acts as an anode.

The electrolytic treatment step is performed to remove chromium oxide formed in the first heating step from the surface layer. Therefore, the steel sheet acts as an anode for electrolytic treatment. A Coulomb density of less than 1.0 C/dm² results in insufficient removal of chromium oxide. A Coulomb density of more than 400.0 A/dm² results in greatly increased costs. Thus, the Coulomb density is 1.0 to 400.0 C/dm². A short electrolytic treatment time may result in insufficient removal of chromium oxide formed at grain boundaries in the steel sheet and poor adhesion of the coating. The electrolytic treatment time is preferably 2 seconds or more, more preferably 5 seconds or more to further improve the adhesion of the coating. Although the electrolytic treatment time has no particular upper limit, a long treating time results in high costs, and therefore the electrolytic treatment time is preferably 60 seconds or less.

Examples of the alkaline aqueous solution used in the electrolytic treatment step include aqueous solutions containing NaOH, Ca(OH)₂, or KOH.

Second Heating Step

The second heating step includes holding the resultant steel sheet after the electrolytic treatment step at a temperature of 650° C. to 900° C. or 650° C. to 900° C. for 15 to 300 seconds in an atmosphere having a H₂ concentration of 0.05% to 25.0% by volume and a dew point of 0° C. or less. The second heating step is performed to facilitate coating (particularly hot-dip coating) of the steel sheet.

The H₂ concentration should be high enough to suppress oxidation of Fe and is 0.05% by volume or more. A H₂ concentration of more than 25.0% by volume results in increased costs. Thus, the H₂ concentration is 25.0% by volume or less.

A dew point of more than 0° C. results in oxidation of Fe. Thus, the dew point is 0° C. or less. The dew point has no particularly lower limit. The dew point is preferably −60° C. or more in terms of industrial practice.

A steel sheet temperature of less than 650° C. results in no activation of the surface of the steel sheet and poor molten zinc wettability. A steel sheet temperature of more than 900° C. results in formation of an oxide of Cr on the surface during annealing, formation of a surface layer containing chromium oxide, and poor wettability of the steel sheet with molten zinc. Thus, the heating temperature of the steel sheet (steel sheet temperature) is a temperature of 650° C. to 900° C. In the second heating step, the steel sheet may be held at a constant temperature or at varying temperatures.

A holding time of less than 15 seconds results in insufficient activation of the surface of the steel sheet. A holding time of 300 seconds or more results in formation of an oxide of Cr on the surface of the steel sheet again, formation of a surface layer containing chromium oxide, and poor molten zinc wettability. Thus, the holding time is 15 to 300 seconds. Coating Treatment Step

For example, the coating treatment step includes cooling the steel sheet after the treatment described above and immersing the steel sheet in a hot-dip galvanizing bath to perform hot-dip galvanizing.

For production of hot-dip galvanized steel sheets, the bath temperature preferably is 440° C. to 550° C., and the concentration of Al in the galvanizing bath preferably is 0.13% to 0.24%. The symbol “%” with respect to the Al concentration refers to “% by mass”.

A bath temperature of less than 440° C. may be inappropriate because temperature variations in the bath may cause solidification of Zn in a low-temperature portion. A bath temperature of more than 550° C. may result in rapid evaporation from the bath and deposition of vaporized Zn on the inner side of the furnace, thereby causing operational problems. This also tends to result in over-alloying because alloying proceeds during coating.

In the production of a hot-dip galvanized steel sheet, when the concentration of Al in the bath is less than 0.14%, this may result in poor adhesion of the coating due to Fe-Zn alloying.

When the concentration of Al in the bath is more than 0.24%, aluminum oxide may cause a defect.

Other Steps

A method of producing a high-strength hot-dip galvanized steel sheet may include other steps, provided that these steps do not have a negative impact. For example, another step may be performed between the steps, before the first heating step, or after the coating treatment step. A specific example that includes another step will be described below in each of the second example and thereafter. Another step is not limited to the step described in each of the second example and examples thereafter.

Second Example

First, a steel sheet that is used as a raw material in a production method according to a second example will be described below. The steel sheet used as a raw material contains C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 8.00%, P: 0.100% or less, S: 0.0100% or less, and Al: 0.100% or less on a mass percent basis, and the remainder is Fe and incidental impurities. The steel sheet may further contain at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0.0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. The steel sheet may further contain at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, N: 0.0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.

The components other than Mn are the same as in the first example and will not be described. The method of producing the steel sheet used as a raw material is also the same as in the first example and will not be described. Mn: 8.00% or less

Like Cr, Mn is oxidized on the surface of the steel sheet in the annealing step and forms a surface layer containing manganese oxide. Because use of Cr increases production costs, Mn, which is similar to Cr in the effects on the material quality, is often added together with Cr. Manganese oxide cannot be removed by alkaline electrolytic treatment, which can remove chromium oxide. However, oxides containing Mn are soluble in acids and can therefore be removed by pickling the surface of the steel sheet after the electrolytic treatment.

Mn is an austenite formation element and effective in securing the strength of annealed sheets. The Mn content is preferably 0.80% or more to secure the strength. However, a Mn content of more than 8.00% may result in insufficient removal of manganese oxide from the surface by pickling. A Mn content of more than 8.00% results in oxidation of a large amount of Mn on the surface of the steel sheet during reannealing, formation of a surface layer containing a large amount of oxide, and a poor appearance of the coating. Thus, the Mn content is 8.00% or less.

A production method according to the second example will be described below. The production method according to the second example includes a first heating step, a cooling step, an electrolytic treatment step, a pickling step after the electrolytic treatment, a second heating step, and a coating treatment step.

Unlike the production method in the first example, the production method in the second example includes the pickling step after the electrolytic treatment. The first heating step, cooling step, electrolytic treatment step, second heating step, and coating treatment step are the same as in the first example and will not be described.

Pickling Step After Electrolytic Treatment

The pickling step after the electrolytic treatment includes, before the second heating step, pickling the surface of the steel sheet after the electrolytic treatment step such that the pickling weight loss is 0.05 to 5 g/m² on an Fe basis. This step is performed to clean the surface of the steel sheet. This step is also performed to remove oxides formed on the surface of the steel sheet in the first heating step and are soluble in acids.

A pickling weight loss of less than 0.05 g/m² on an Fe basis may result in insufficient removal of oxides. A pickling weight loss of more than 5 g/m² may result in dissolution of not only oxides on the surface layer of the steel sheet but also an inner portion of the steel sheet where Cr concentration has been decreased, and thus may fail to suppress formation of chromium oxide in the second heating step. Thus, the pickling weight loss is 0.05 to 5 g/m² on an Fe basis.

Third Example

First, a steel sheet that is used as a raw material in a production method according to a third example will be described below. The steel sheet used as a raw material contains C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 3.00%, Mn: 8.00% or less, P: 0.100% or less, S: 0.0100% or less, and Al: 0.100% or less on a mass percent basis, and the remainder is Fe and incidental impurities. The steel sheet may further contain at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0.0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. The steel sheet may further contain at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, N: 0.0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.

The above described steel sheet used in the third example is substantially the same as the steel sheet used as a raw material in the production method in the second example. Thus, the components other than Cr and the production method of the steel sheet will not be described. Cr: 0.10% to 3.00%

Cr is an austenite formation element and effective in securing the strength of annealed sheets. The Cr content is 0.10% or more to secure the strength. A Cr content of more than 3.00% results in insufficient removal of chromium oxide from the surface of the steel sheet even utilizing the third example, resulting in a poor appearance of the coating. Thus, the Cr content is 3.00% or less.

A production method according to the third example will be described below. The production method according to the third example includes a first heating step, a cooling step, a pickling step before electrolytic treatment, an electrolytic treatment step, a second heating step, and a coating treatment step. The first heating step, cooling step, electrolytic treatment step, second heating step, and coating treatment step are the same as in the first example and will not be described.

Pickling Step Before Electrolytic Treatment

The pickling step before electrolytic treatment includes, before the electrolytic treatment step, pickling a surface of the steel sheet after the cooling step such that the pickling weight loss is 0.05 to 5 g/m² on an Fe basis. This step is performed to clean the surface of the steel sheet. This step is also performed to remove oxides that are formed on the surface of the steel sheet in the first heating step and are soluble in acids.

A surface layer containing manganese oxide and chromium oxide is formed on the surface of the steel sheet after annealing. Manganese oxide is formed closer to the outermost side, and chromium oxide is formed closer to the steel sheet. Thus, chromium oxide can be more effectively removed in the electrolytic treatment step by pickling the surface of the steel sheet before the electrolytic treatment step to remove manganese oxide.

A pickling weight loss of less than 0.05 g/m² on an Fe basis may result in insufficient removal of oxides. A pickling weight loss of more than 5 g/m² may result in dissolution of not only oxides on the surface layer of the steel sheet but also an inner portion of the steel sheet that has a low Cr concentration, thus failing to suppress formation of chromium oxide in the second heating step. Thus, the pickling weight loss is 0.05 to 5 g/m² on an Fe basis.

The production method may include both the pickling step after the electrolytic treatment described in the production method according to the second example and the pickling step before electrolytic treatment.

Fourth Example

A production method according to a fourth example further includes an alloying treatment step after the coating treatment step of the first, second, or third example.

In the production method in the fourth example, the alloying treatment step is preferably performed after the following coating treatment step.

Coating Treatment Step

For production of high-strength galvannealed steel sheets, the bath temperature preferably is 440° C. to 550° C., and the concentration of Al in the galvanizing bath preferably is 0.10% to 0.20%.

A bath temperature of less than 440° C. may be inappropriate because variations in the bath temperature may cause solidification of Zn in a low-temperature portion. A bath temperature of more than 550° C. may result in rapid evaporation from the bath and deposition of vaporized Zn on the furnace, thereby causing operational problems. A bath temperature of more than 550° C. also tends to result in over-alloying because alloying proceeds during coating.

When the concentration of Al in the bath is less than 0.10%, this may result in formation of a large amount of phase and a poor powdering property. When the concentration of Al in the bath is more than 0.20%, Fe-Zn alloying may not proceed.

Alloying Treatment Step

Although the conditions for the alloying treatment are not particularly limited, the alloying treatment temperature is most preferably more than 460° C. and less than 580° C. An alloying treatment temperature of 460° C. or less results in slow alloying. An alloying treatment temperature of 580° C. or more causes excessive formation of a hard and brittle Zn-Fe alloy layer at an interface with the base metal due to over-alloying and results in poor adhesion of the coating.

EXAMPLES EXAMPLE 1

Steel having the composition listed in Table 1, the remainder being Fe and incidental impurities, was produced in a converter and formed into a slab in a continuous casting process. The slab was heated to 1200° C., was then hot-rolled to a thickness of 2.3 to 4.5 mm, and coiled. The hot-rolled steel sheet was then pickled and, if necessary, cold-rolled. The first heating step, electrolytic treatment step, and second heating step were then performed under the heat-treatment conditions listed in Table 2 (Tables 2-1 and 2-2 are collectively referred to as Table 2) or Table 3 (Tables 3-1 and 3-2 are collectively referred to as Table 3) in a furnace in which the atmosphere could be adjusted (the temperature in the first heating step and the temperature in the second heating step were in the temperature ranges around the temperatures listed in the tables (the temperatures listed in the tables ±20° C.), respectively). Hot-dip galvanizing treatment was subsequently performed in a Zn bath containing 0.13% to 0.24% Al to produce a hot-dip galvanized steel sheet. After hot-dip galvanizing treatment was performed in a Zn bath containing 0.10% to 0.20% Al, alloying treatment was performed under the conditions listed in Table 2 to produce a galvannealed steel sheet.

The surface appearance and adhesion of the coating of the hot-dip galvanized steel sheet and galvannealed steel sheet thus produced were examined by the following methods. Surface Appearance

The steel sheets were visually inspected for appearance deficiencies such as ungalvanized surfaces and pinholes. Steel sheets having no appearance deficiencies were rated as good (circle). Steel sheets having generally good appearances with a few appearance deficiencies were rated as fair (triangle). Steel sheets having appearance deficiencies were rated as poor (cross).

Adhesion of the Coating

Adhesion of the coating of galvannealed steel sheets was evaluated in terms of powdering resistance. More specifically, a cellophane adhesive tape was applied to a galvannealed steel sheet. The surface to which the tape was applied was bent at an angle of 90 degrees and was bent back. A cellophane adhesive tape having a width of 24 mm was applied to and pressed against the inside of a processed portion (compressed side) parallel to the bent-processed portion and was then peeled off. The amount of zinc adhered to a portion of the cellophane adhesive tape having a length of 40 mm was measured as a Zn count using fluorescent X-rays. The Zn count per unit length (1 m) was rated according to the following criteria. The rank 1 is good (circle), the rank 2 is fair (triangle), and the rank 3 or more are poor (cross).

Fluorescent X-rays count rank

-   -   0 or more and less than 500:1 (good)     -   500 or more and less than 1000:2     -   1000 or more and less than 2000:3     -   2000 or more and less than 3000:4     -   3000 or more: 5 (poor)

Unalloyed hot-dip galvanized steel sheets were subjected to a ball impact test. Adhesion of the coating was evaluated by peeling off a processed portion with a cellophane adhesive tape and by visually inspecting the processed portion for peeling of the coated layer. In the ball impact test, the mass of the ball was 1.8 kg, and the drop height was 100 cm.

-   -   Circle: No peeling of the coated layer     -   Cross: Peeling of the coated layer

Table 2 shows the evaluation results.

TABLE 1 (mass %) Steel type symbol C Si Mn P S Al N Cr Mo B A 0.089 0.15 2.66 0.005 0.0010 0.031 0.0036 0.61 0.09 0.0009 B 0.086 0.16 2.58 0.008 0.0010 0.030 0.0031 0.58 0.10 0.0010 C 0.091 0.16 2.34 0.009 0.0008 0.033 0.0029 0.14 0.34 0.0010 D 0.083 0.13 2.32 0.006 0.0023 0.032 0.0030 1.35 0.08 0.0008 E 0.085 0.05 2.53 0.004 0.0008 0.024 0.0033 1.86 0.21 0.0010 F 0.113 0.29 2.58 0.004 0.0018 0.003 0.0028 2.84 0.15 0.0009 G 0.089 0.14 0.81 0.007 0.0015 0.032 0.0029 0.57 0.10 0.0009 H 0.087 0.15 2.31 0.004 0.0011 0.033 0.0030 0.59 0.09 0.0008 I 0.083 0.17 4.57 0.004 0.0009 0.031 0.0030 0.35 0.25 0.0010 J 0.085 0.17 7.68 0.007 0.0016 0.029 0.0030 0.62 0.11 0.0011 K 0.088 0.01 2.19 0.009 0.0018 0.030 0.0031 0.59 0.08 0.0007 L 0.116 0.48 2.43 0.006 0.0007 0.028 0.0030 0.64 0.02 0.0014 M 0.086 0.78 2.88 0.005 0.0019 0.029 0.0032 0.72 0.09 0.0013 N 0.089 0.15 2.28 0.007 0.0026 0.031 0.0029 0.81 0.11 0.0012 O 0.084 0.16 2.34 0.006 0.0023 0.029 0.0036 0.54 0.12 0.0010 P 0.105 0.12 2.74 0.010 0.0018 0.027 0.0035 0.25 0.10 0.0008 Q 0.089 0.05 4.85 0.007 0.0010 0.031 0.0031 0.48 0.12 0.0009 R 0.078 0.23 2.68 0.008 0.0031 0.030 0.0029 0.18 0.90 0.0007 S 0.086 0.55 10.80  0.005 0.0012 0.032 0.0030 1.42 0.12 0.0010 T 0.091 0.16 2.35 0.005 0.0024 0.028 0.0032 3.45 0.12 0.0009 U 0.089 2.84 2.54 0.009 0.0027 0.030 0.0035 0.08 0.90 0.0011 Steel type symbol Ti Nb Ni Cu V Sb Sn Ca REM A 0.023 0.039 — — — — — — — B 0.018 0.032 — — — — — — — C 0.021 0.042 — — — — — — — D 0.033 0.035 — — — — — — — E 0.025 0.032 — — — — — — — F 0.024 0.038 — — — — — — — G 0.029 0.036 — — — — — — — H 0.035 0.041 — — — — — — — I 0.029 0.040 — — — — — — — J 0.024 0.040 — — — — — — — K 0.022 0.036 — — 0.058 — — — — L 0.019 0.042 — 0.05 — — — — — M 0.031 0.042 — — — — — — — N 0.022 0.037 — — — — 0.03 — — O 0.033 0.035 0.22 — — — — — — P 0.015 0.042 — — — — — 0.0008 — Q 0.031 0.038 — — — — — — 0.001 R 0.027 0.040 — — — 0.02 — — — S 0.022 0.038 — — — — — — — T 0.023 0.044 — — — — — — — U 0.029 0.038 — — — — — — —

TABLE 2-1 (Example) Electrolytic First heating step treatment Second heating step Coating Dew Heating Holding Charge Dew Heating Holding treatment step H₂ point temperature time density H₂ point temperature time Al concentration No Steel (%) (° C.) (° C.) (s) (C/dm²) (%) (° C.) (° C.) (s) (%)  1 A 5.0 −40 850 300 100  10.0 −35 800 200 0.137  2 A 5.0 −40 800 300 20 5.0 −35 800 200 0.191  3 A 5.0 −10 800 200 50 15.0 −40 800 200 0.138  4 A 5.0   10 870 300 80 5.0 −35 850 100 0.130  5 A 5.0 −40 850 300 380  10.0 −35 800 200 0.132  6 A 5.0 −40 850 200 350  15.0 −35 820 150 0.195  7 A 5.0 −40 850 300  2 10.0 −35 800 200 0.138  8 A 5.0 −35 850 300  4 15.0 −35 820 150 0.187  9 A 5.0 −40 830 300   0.5 10.0 −35 800 200 0.137 10 A 5.0 −40 820 300    0.01 10.0 −40 800 200 0.189 11 A 5.0 −40 870 300 25 10.0 −35 800 200 0.126 12 A 5.0 −40 760 300 10 10.0 −35 800 200 0.136 13 A 3.0 −35 850  30 25 5.0 −35 790  50 0.189 14 A 5.0 −40 650 300 100  10.0 −35 800 200 0.138 15 A 5.0 −40 800 800 100  10.0 −35 800 200 0.132 16 A 5.0 −40 830  5 100  10.0 −35 800 200 0.190 17 A 5.0 −40 860 300 20 10.0 −35 880 200 0.190 18 A 10.0 −45 750 350 30 10.0 −35 700 200 0.138 19 A 5.0 −40 850 300 100  10.0 −35 950 200 0.133 20 A 5.0 −40 830 300 100  10.0 −35 550 200 0.193 21 B 10.0 −35 850 300 10 5.0 −35 790 200 0.137 22 B 5.0 −35 860 300 100  5.0 −35 800  20 0.138 23 B 10.0 −40 860 250 50 10.0 −35 800 280 0.190 24 B 5.0 −40 850 250 150  5.0 −35 800  10 0.188 25 B 5.0 −40 830 400 100  5.0 −35 800 850 0.189 26 C 5.0 −35 850 400 250  10.0 −35 800 250 0.141 27 C 5.0 −35 850 400 10 10.0 −40 800 250 0.190 28 D 5.0 −35 830 300 25 10.0 −35 800 200 0.138 29 D 5.0 −35 850 300 50 10.0 −35 800 100 0.190 30 D 15.0 −40 800 250 230  5.0 −35 790 100 0.134 31 E 5.0 −40 840 200 50 10.0 −35 800 200 0.131 32 E 5.0 −40 840 300 10 15.0 −35 820 200 0.192 33 F 5.0 −40 850 300 10 10.0 −35 800 200 0.190 Alloying treatment step Alloying temperature Surface No (° C.) appearance Adhesion Product Remarks  1 510 ◯ ◯ GA Example  2 — ◯ ◯ GI Example  3 500 Δ ◯ GA Example  4 490 X Δ GA Comparative example  5 510 ◯ ◯ GA Example  6 — ◯ ◯ GI Example  7 470 Δ ◯ GA Example  8 — Δ ◯ GI Example  9 520 X X GA Comparative example 10 — X X GI Comparative example 11 480 ◯ ◯ GA Example 12 480 Δ ◯ GA Example 13 — ◯ ◯ GI Example 14 470 X X GA Comparative example 15 480 Δ X GA Comparative example 16 — X X GI Comparative example 17 — ◯ Δ GI Example 18 500 ◯ ◯ GA Example 19 480 X X GA Comparative example 20 — Δ X GI Comparative example 21 520 ◯ ◯ GA Example 22 530 ◯ Δ GA Example 23 — ◯ ◯ GI Example 24 — X X GI Comparative example 25 — X X GI Comparative example 26 480 ◯ ◯ GA Example 27 — ◯ ◯ GI Example 28 490 ◯ ◯ GA Example 29 — ◯ ◯ GI Example 30 510 ◯ ◯ GA Example 31 490 ◯ Δ GA Example 32 — ◯ Δ GI Example 33 — X X GI Comparative example

TABLE 2-2 (Example) Electrolytic First heating step treatment Second heating step Coating Dew Heating Holding Charge Dew Heating Holding treatment step H₂ point temperature time density H₂ point temperature time Al concentration No Steel (%) (° C.) (° C.) (s) (C/dm²) (%) (° C.) (° C.) (s) (%) 34 F 10.0 −40 850 250 50 10.0 −35 820 200 0.138 35 G 5.0 −40 850 300 15 5.0 −35 800 200 0.137 36 G 5.0 −40 810 300 50 10.0 −45 780 150 0.188 37 G 10.0 −40 790 400 15 5.0 −35 790 200 0.195 38 G 10.0 −40 810 250 100 15.0 −35 800 150 0.135 39 H 5.0 −40 850 320 20 10.0 −35 800 200 0.189 40 H 5.0 −40 850 300 15 10.0 −20 800 200 0.137 41 H 5.0 −40 800 300 10 10.0 −35 860 200 0.189 42 H 5.0 −40 800 300 200 15.0 −35 800 300 0.137 43 H 10.0 −50 800 100 100 5.0 −35 810 250 0.190 44 I 5.0 −40 850 300 15 10.0 −35 800 200 0.137 45 I 10.0 −35 840 150 60 10.0 −35 800 200 0.189 46 J 5.0 −40 850 300 100 10.0 −35 800 200 0.137 47 J 5.0 −35 820 300 120 10.0 −35 800 200 0.130 48 J 5.0 −40 830 300 180 6.0 −35 800 200 0.189 49 K 5.0 −40 840 300 10 10.0 −35 800 200 0.136 50 K 10.0 −35 850 200 50 10.0 −40 830 100 0.192 51 L 5.0 −35 850 300 20 5.0 −40 800 100 0.189 52 L 1.0 −40 830 300 10 10.0 −35 800 200 0.129 53 L 10.0 −35 850 250 50 1.0 −35 800 200 0.137 54 L 5.0 −35 870 550 10 10.0 −40 800 250 0.130 55 L 5.0 −35 870 680 10 5.0 −40 800 250 0.190 56 L 5.0 −35 860 300 50 10.0 −10 800 150 0.189 57 L 5.0 −35 850 300 50 10.0   10 850 150 0.137 58 M 5.0 −40 850 300 200 10.0 −45 800 200 0.138 59 N 5.0 −40 840 350 150 5.0 −35 780 200 0.193 60 O 5.0 −35 830 300 12 10.0 −35 800 200 0.138 61 O 10.0 −35 800 250 50 15.0 −30 780 150 0.190 62 P 5.0 −40 820 300 100 10.0 −35 800 200 0.131 63 Q 5.0 −40 820 300 100 10.0 −35 800 200 0.138 64 R 5.0 −40 820 300 100 10.0 −35 800 200 0.137 65 S 5.0 −35 850 250 50 10.0 −35 800 250 0.190 66 T 5.0 −40 850 300 12 10.0 −35 800 200 0.138 67 U 5.0 −40 850 300 12 10.0 −35 800 200 0.137 Alloying treatment step Alloying temperature Surface No (° C.) appearance Adhesion Product Remarks 34 520 X X GA Comparative example 35 480 ◯ ◯ GA Example 36 — ◯ ◯ GI Example 37 — ◯ ◯ GI Example 38 480 ◯ ◯ GA Example 39 — ◯ ◯ GI Example 40 480 ◯ ◯ GA Example 41 — ◯ ◯ GI Example 42 480 ◯ ◯ GA Example 43 — X X GI Comparative example 44 490 ◯ Δ GA Example 45 — ◯ Δ GI Example 46 540 X Δ GA Comparative example 47 520 X X GA Comparative example 48 — X X GI Comparative example 49 540 ◯ ◯ GA Example 50 510 ◯ ◯ GI Example 51 — ◯ ◯ GI Example 52 490 Δ ◯ GA Example 53 520 ◯ Δ GA Example 54 500 Δ ◯ GA Example 55 — X Δ GI Comparative example 56 — Δ ◯ GI Example 57 480 X Δ GA Comparative example 58 490 ◯ ◯ GA Example 59 — ◯ ◯ GI Example 60 510 ◯ ◯ GA Example 61 — ◯ ◯ GI Example 62 520 ◯ ◯ GA Example 63 540 ◯ ◯ GA Example 64 510 ◯ ◯ GA Example 65 — X X GI Comparative example 66 480 X X GA Comparative example 67 490 X X GA Comparative example

The high-strength hot-dip galvanized steel sheets and high-strength galvannealed steel sheets according to our examples had a good surface appearance and high adhesion. In contrast, the comparative examples had poor surface appearance and poor adhesion of the coating.

EXAMPLE 2

Hot-dip galvanized steel sheets and galvannealed steel sheets were produced from the steel having the composition listed in Table 1, the remainder being Fe and incidental impurities in the same manner as in Example 1, except that the conditions listed in Table 3 were employed. Evaluation was performed in the same way as in Example 1. Table 3 shows the evaluation results.

TABLE 3-1 (Example) Electrolytic Pickling step First heating step treatment after electrolytic Second heating step Dew Heating Holding Charge treatment Dew Heating Holding H₂ point temperature time density Weight loss H₂ point temperature time No Steel (%) (° C.) (° C.) (s) (C/dm²) (g/m²) (%) (° C.) (° C.) (s) 68 A 5.0 −40 850 300 100  0.45 10.0 −35 800 200 69 A 5.0 −40 800 300 20 0.34 5.0 −35 800 200 70 A 5.0 −10 800 200 50 0.42 15.0 −40 800 200 71 A 5.0   10 870 300 80 0.86 5.0 −35 850 100 72 A 5.0 −40 850 300 380  0.56 10.0 −35 800 200 73 A 5.0 −40 850 200 350  0.48 15.0 −35 820 150 74 A 5.0 −40 850 300  2 0.05 10.0 −35 800 200 75 A 5.0 −35 850 300  4 0.25 15.0 −35 820 150 76 A 5.0 −40 830 300   0.5 0.07 10.0 −35 800 200 77 A 5.0 −40 820 300    0.01 0.38 10.0 −40 800 200 78 A 5.0 −40 870 300 25 0.06 10.0 −35 800 200 79 A 5.0 −40 760 300 10 0.12 10.0 −35 800 200 80 A 3.0 −35 850  30 25 0.42 5.0 −35 790  50 81 A 5.0 −40 650 300 100  0.12 10.0 −35 800 200 82 A 5.0 −40 800 800 100  0.21 10.0 −35 800 200 83 A 5.0 −40 830  5 100  0.06 10.0 −35 800 200 84 A 5.0 −40 860 300 20 0.28 10.0 −35 880 200 85 A 10.0 −45 750 350 30 0.08 10.0 −35 700 200 86 A 5.0 −40 850 300 100  0.28 10.0 −35 950 200 87 A 5.0 −40 830 300 100  0.11 10.0 −35 550 200 88 B 10.0 −35 850 300 10 0.23 5.0 −35 790 200 89 B 5.0 −35 860 300 100  0.15 5.0 −35 800  20 90 B 10.0 −40 860 250 50 0.33 10.0 −35 800 280 91 B 5.0 −40 850 250 150  0.46 5.0 −35 800  10 92 B 5.0 −40 830 400 100  0.27 5.0 −35 800 850 93 C 5.0 −35 850 400 250  0.05 10.0 −35 800 250 94 C 5.0 −35 850 400 10 0.16 10.0 −40 800 250 95 D 5.0 −35 830 300 25 0.43 10.0 −35 800 200 96 D 5.0 −35 850 300 50 0.08 10.0 −35 800 100 97 D 15.0 −40 800 250 230  0.04 5.0 −35 790 100 98 E 5.0 −40 840 200 50 1.25 10.0 −35 800 200 99 E 5.0 −40 840 300 10 0.85 15.0 −35 820 200 100  F 5.0 −40 850 300 10 0.12 10.0 −35 800 200 101  F 10.0 −40 850 250 50 0.25 10.0 −35 820 200 Alloying treatment Coating step treatment step Alloying Al concentration temperature Surface No (%) (° C.) appearance Adhesion Product Remarks 68 0.137 510 ◯ ◯ GA Example 69 0.191 — ◯ ◯ GI Example 70 0.138 500 Δ ◯ GA Example 71 0.130 490 X Δ GA Comparative example 72 0.132 510 ◯ ◯ GA Example 73 0.195 — ◯ ◯ GI Example 74 0.138 470 Δ ◯ GA Example 75 0.187 — Δ ◯ GI Example 76 0.137 520 X X GA Comparative example 77 0.189 — X X GI Comparative example 78 0.126 480 ◯ ◯ GA Example 79 0.136 480 Δ ◯ GA Example 80 0.189 — ◯ ◯ GI Example 81 0.138 470 X X GA Comparative example 82 0.132 480 Δ X GA Comparative example 83 0.190 — X X GI Comparative example 84 0.190 — ◯ Δ GI Example 85 0.138 500 ◯ ◯ GA Example 86 0.133 480 X X GA Comparative example 87 0.193 — Δ X GI Comparative example 88 0.137 520 ◯ ◯ GA Example 89 0.138 530 ◯ Δ GA Example 90 0.190 — ◯ ◯ GI Example 91 0.188 — X X GI Comparative example 92 0.189 — X X GI Comparative example 93 0.141 480 ◯ ◯ GA Example 94 0.190 — ◯ ◯ GI Example 95 0.138 490 ◯ ◯ GA Example 96 0.190 — ◯ ◯ GI Example 97 0.134 510 ◯ ◯ GA Example 98 0.131 490 ◯ Δ GA Example 99 0.192 — ◯ Δ GI Example 100  0.190 — X X GI Comparative example 101  0.138 520 X X GA Comparative example

TABLE 3-2 (Example) Electrolytic Pickling step after First heating step treatment electrolytic Second heating step Dew Heating Holding Charge treatment Dew Heating H₂ point temperature time density Weight loss H₂ point temperature Holding time No Steel (%) (° C.) (° C.) (s) (C/dm²) (g/m²) (%) (° C.) (° C.) (s) 102 G 5.0 −40 850 300 15 0.05 5.0 −35 800 200 103 G 5.0 −40 810 300 50 0.12 10.0 −45 780 150 104 G 10.0 −40 790 400 15 0.38 5.0 −35 790 200 105 G 10.0 −40 810 250 100 0.87 15.0 −35 800 150 106 H 5.0 −40 850 320 20 0.42 10.0 −35 800 200 107 H 5.0 −40 850 300 15 0.07 10.0 −20 800 200 108 H 5.0 −40 800 300 10 4.8 10.0 −35 860 200 109 H 5.0 −40 800 300 200 1.83 15.0 −35 800 300 110 H 10.0 −50 800 100 100 0.29 5.0 −35 810 250 111 I 5.0 −40 850 300 15 0.35 10.0 −35 800 200 112 I 10.0 −35 840 150 60 0.58 10.0 −35 800 200 113 J 5.0 −40 850 300 100 0.26 10.0 −35 800 200 114 J 5.0 −35 820 300 120 0.33 10.0 −35 800 200 115 J 5.0 −40 830 300 180 0.48 6.0 −35 800 200 116 K 5.0 −40 840 300 10 0.12 10.0 −35 800 200 117 K 10.0 −35 850 200 50 0.38 10.0 −40 830 100 118 L 5.0 −35 850 300 20 0.52 5.0 −40 800 100 119 L 1.0 −40 830 300 10 0.08 10.0 −35 800 200 120 L 10.0 −35 850 250 50 0.15 1.0 −35 800 200 121 L 5.0 −35 870 550 10 0.28 10.0 −40 800 250 122 L 5.0 −35 870 680 10 0.18 5.0 −40 800 250 123 L 5.0 −35 860 300 50 0.34 10.0 −10 800 150 124 L 5.0 −35 850 300 50 0.18 10.0   10 850 150 125 M 5.0 −40 850 300 200 0.27 10.0 −45 800 200 126 N 5.0 −40 840 350 150 0.22 5.0 −35 780 200 127 O 5.0 −35 830 300 12 0.35 10.0 −35 800 200 128 O 10.0 −35 800 250 50 0.18 15.0 −30 780 150 129 P 5.0 −40 820 300 100 0.45 10.0 −35 800 200 130 Q 5.0 −40 820 300 100 0.45 10.0 −35 800 200 131 R 5.0 −40 820 300 100 0.45 10.0 −35 800 200 132 S 5.0 −35 850 250 50 0.15 10.0 −35 800 250 133 T 5.0 −40 850 300 12 0.04 10.0 −35 800 200 134 U 5.0 −40 850 300 12 0.04 10.0 −35 800 200 Coating treatment Alloying treatment step step Al concentration Alloying temperature Surface No (%) (° C.) appearance Adhesion Product Remarks 102 0.137 480 ◯ ◯ GA Example 103 0.188 — ◯ ◯ GI Example 104 0.195 — ◯ ◯ GI Example 105 0.135 480 ◯ ◯ GA Example 106 0.189 — ◯ ◯ GI Example 107 0.137 480 ◯ Δ GA Example 108 0.189 — ◯ Δ GI Example 109 0.137 480 ◯ ◯ GA Example 110 0.190 — X X GI Comparative example 111 0.137 490 ◯ ◯ GA Example 112 0.189 — ◯ ◯ GI Example 113 0.137 540 Δ ◯ GA Example 114 0.130 520 Δ Δ GA Example 115 0.189 — Δ Δ GI Example 116 0.136 540 ◯ ◯ GA Example 117 0.192 510 ◯ ◯ GI Example 118 0.189 — ◯ ◯ GI Example 119 0.129 490 Δ ◯ GA Example 120 0.137 520 ◯ Δ GA Example 121 0.130 500 Δ ◯ GA Example 122 0.190 — X Δ GI Comparative example 123 0.189 — Δ ◯ GI Example 124 0.137 480 X Δ GA Comparative example 125 0.138 490 ◯ ◯ GA Example 126 0.193 — ◯ ◯ GI Example 127 0.138 510 ◯ ◯ GA Example 128 0.190 — ◯ ◯ GI Example 129 0.131 520 ◯ ◯ GA Example 130 0.138 540 ◯ ◯ GA Example 131 0.137 510 ◯ ◯ GA Example 132 0.190 — X X GI Comparative example 133 0.138 480 X X GA Comparative example 134 0.137 490 X X GA Comparative example

The hot-dip galvanized steel sheets and galvannealed steel sheets according to our examples had a good surface appearance and high adhesion. In contrast, the comparative examples had poor surface appearance and poor adhesion of the coating.

EXAMPLE 3

Hot-dip galvanized steel sheets and galvannealed steel sheets were produced from the steel having the composition listed in Table 1, the remainder being Fe and incidental impurities in the same manner as in Example 1, except that the conditions listed in Table 4 (Tables 4-1 and 4-2 are collectively referred to as Table 4) were employed. Evaluation was performed in the same way as in Example 1. Table 4 shows the evaluation results.

TABLE 4-1 (Example) Pickling step Electrolytic Alloying First heating step before electrolytic treatment Second heating step Coating treatment step Dew Heating treatment Charge Dew Heating Holding treatment step Alloying H₂ point temperature Holding time Weight loss density H₂ point temperature time Al concentration temperature Surface No Steel (%) (° C.) (° C.) (s) (g/m²) (C/dm²) (%) (° C.) (° C.) (s) (%) (° C.) appearance Adhesion Product Remarks 135 A 5.0 −40 850 300 0.45 100  10.0 −35 800 200 0.137 510 ◯ ◯ GA Example 136 A 5.0 −40 800 300 0.34 20 5.0 −35 800 200 0.191 — ◯ ◯ GI Example 137 A 5.0 −10 800 200 0.42 50 15.0 −40 800 200 0.138 500 Δ ◯ GA Example 138 A 5.0  10 870 300 0.86 80 5.0 −35 850 100 0.130 490 X Δ GA Comparative example 139 A 5.0 −40 850 300 0.56 380  10.0 −35 800 200 0.132 510 ◯ ◯ GA Example 140 A 5.0 −40 850 200 0.48 350  15.0 −35 820 150 0.195 — ◯ ◯ GI Example 141 A 5.0 −40 850 300 0.05  2 10.0 −35 800 200 0.138 470 Δ ◯ GA Example 142 A 5.0 −35 850 300 0.25  4 15.0 −35 820 150 0.187 — Δ ◯ GI Example 143 A 5.0 −40 830 300 0.07   0.5 10.0 −35 800 200 0.137 520 X X GA Comparative example 144 A 5.0 −40 820 300 0.38    0.01 10.0 −40 800 200 0.189 — X X GI Comparative example 145 A 5.0 −40 870 300 0.06 25 10.0 −35 800 200 0.126 480 ◯ ◯ GA Example 146 A 5.0 −40 760 300 0.12 10 10.0 −35 800 200 0.136 480 Δ ◯ GA Example 147 A 3.0 −35 850  30 0.42 25 5.0 −35 790  50 0.189 — ◯ ◯ GI Example 148 A 5.0 −40 650 300 0.12 100  10.0 −35 800 200 0.138 470 X X GA Comparative example 149 A 5.0 −40 800 800 0.21 100  10.0 −35 800 200 0.132 480 Δ X GA Comparative example 150 A 5.0 −40 830  5 0.06 100  10.0 −35 800 200 0.190 — X X GI Comparative example 151 A 5.0 −40 860 300 0.28 20 10.0 −35 880 200 0.190 — ◯ Δ GI Example 152 A 10.0 −45 750 350 0.08 30 10.0 −35 700 200 0.138 500 ◯ ◯ GA Example 153 A 5.0 −40 850 300 0.28 100  10.0 −35 950 200 0.133 480 X X GA Comparative example 154 A 5.0 −40 830 300 0.11 100  10.0 −35 550 200 0.193 — Δ X GI Comparative example 155 B 10.0 −35 850 300 0.23 10 5.0 −35 790 200 0.137 520 ◯ ◯ GA Example 156 B 5.0 −35 860 300 0.15 100  5.0 −35 800  20 0.138 530 ◯ Δ GA Example 157 B 10.0 −40 860 250 0.33 50 10.0 −35 800 280 0.190 — ◯ ◯ GI Example 158 B 5.0 −40 850 250 0.46 150  5.0 −35 800  10 0.188 — X X GI Comparative example 159 B 5.0 −40 830 400 0.27 100  5.0 −35 800 850 0.189 — X X GI Comparative example 160 C 5.0 −35 850 400 0.05 250  10.0 −35 800 250 0.141 480 ◯ ◯ GA Example 161 C 5.0 −35 850 400 0.16 10 10.0 −40 800 250 0.190 — ◯ ◯ GI Example 162 D 5.0 −35 830 300 0.43 25 10.0 −35 800 200 0.138 490 ◯ ◯ GA Example 163 D 5.0 −35 850 300 0.08 50 10.0 −35 800 100 0.190 — ◯ ◯ GI Example 164 D 15.0 −40 800 250 0.04 230  5.0 −35 790 100 0.134 510 ◯ ◯ GA Example 165 E 5.0 −40 840 200 1.25 50 10.0 −35 800 200 0.131 490 ◯ ◯ GA Example 166 E 5.0 −40 840 300 0.85 10 15.0 −35 820 200 0.192 — ◯ ◯ GI Example 167 F 5.0 −40 850 300 0.12 10 10.0 −35 800 200 0.190 — ◯ Δ GI Example 168 F 10.0 −40 850 250 0.25 50 10.0 −35 820 200 0.138 520 ◯ Δ GA Example

TABLE 4-2 (Example) Pickling step before Electrolytic Alloying First heating step electrolytic treatment Second heating step Coating treatment step Dew Heating Holding treatment Charge Dew Heating Holding treatment step Alloying H₂ point temperature time Weight loss density H₂ point temperature time Al concentration temperature Surface No Steel (%) (° C.) (° C.) (s) (g/m²) (C/dm²) (%) (° C.) (° C.) (s) (%) (° C.) appearance Adhesion Product Remarks 169 G 5.0 −40 850 300 0.05 15 5.0 −35 800 200 0.137 480 ◯ ◯ GA Example 170 G 5.0 −40 810 300 0.12 50 10.0 −35 780 150 0.188 — ◯ ◯ GI Example 171 G 10.0 −40 790 400 0.38 15 5.0 −35 790 200 0.195 — ◯ ◯ GI Example 172 G 10.0 −40 810 250 0.87 100 15.0 −35 800 150 0.135 480 ◯ ◯ GA Example 173 H 5.0 −40 850 320 0.42 20 10.0 −35 800 200 0.189 — ◯ ◯ GI Example 174 H 5.0 −40 850 300 0.07 15 10.0 −20 800 200 0.137 480 ◯ Δ GA Example 175 H 5.0 −40 800 300 4.8 10 10.0 −35 860 200 0.189 — ◯ Δ GI Example 176 H 5.0 −40 800 300 1.83 200 15.0 −35 800 300 0.137 480 ◯ ◯ GA Example 177 H 10.0 −50 800 100 0.29 100 5.0 −35 810 250 0.190 — X X GI Comparative example 178 I 5.0 −40 850 300 0.35 15 10.0 −35 800 200 0.137 490 ◯ ◯ GA Example 179 I 10.0 −35 840 150 0.58 60 10.0 −35 800 200 0.189 — ◯ ◯ GI Example 180 J 5.0 −40 850 300 0.26 100 10.0 −35 800 200 0.137 540 ◯ ◯ GA Example 181 J 5.0 −35 820 300 0.33 120 10.0 −35 800 200 0.130 520 ◯ ◯ GA Example 182 J 5.0 −40 830 300 0.48 180 6.0 −35 800 200 0.189 — ◯ ◯ GI Example 183 K 5.0 −40 840 300 0.12 10 10.0 −35 800 200 0.136 540 ◯ ◯ GA Example 184 K 10.0 −35 850 200 0.38 50 10.0 −40 830 100 0.192 510 ◯ ◯ GI Example 185 L 5.0 −35 850 300 0.52 20 5.0 −40 800 100 0.189 — ◯ ◯ GI Example 186 L 1.0 −40 830 300 0.08 10 10.0 −35 800 200 0.129 490 Δ ◯ GA Example 187 L 10.0 −35 850 250 0.15 50 1.0 −35 800 200 0.137 520 ◯ Δ GA Example 188 L 5.0 −35 870 550 0.28 10 10.0 −40 800 250 0.130 500 Δ ◯ GA Example 189 L 5.0 −35 870 680 0.18 10 5.0 −40 800 250 0.190 — X Δ GI Comparative example 190 L 5.0 −35 860 300 0.34 50 10.0 −10 800 150 0.189 — Δ ◯ GI Example 191 L 5.0 −35 850 300 0.18 50 10.0  10 850 150 0.137 480 X Δ GA Comparative example 192 M 5.0 −40 850 300 0.27 200 10.0 −45 800 200 0.138 490 ◯ ◯ GA Example 193 N 5.0 −40 840 350 0.22 150 5.0 −35 780 200 0.193 — ◯ ◯ GI Example 194 O 5.0 −35 830 300 0.35 12 10.0 −35 800 200 0.138 510 ◯ ◯ GA Example 195 O 10.0 −35 800 250 0.18 50 15.0 −30 780 150 0.190 — ◯ ◯ GI Example 196 P 5.0 −40 820 300 0.45 100 10.0 −35 800 200 0.131 520 ◯ ◯ GA Example 197 Q 5.0 −40 820 300 0.45 100 10.0 −35 800 200 0.138 540 ◯ ◯ GA Example 198 R 5.0 −40 820 300 0.45 100 10.0 −35 800 200 0.137 510 ◯ ◯ GA Example 199 S 5.0 −35 850 250 0.15 50 10.0 −35 800 250 0.190 — X X GI Comparative example 200 T 5.0 −40 850 300 0.04 12 10.0 −35 800 200 0.138 480 X X GA Comparative example 201 U 5.0 −40 850 300 0.04 12 10.0 −35 800 200 0.137 490 X X GA Comparative example

The hot-dip galvanized steel sheets and galvannealed steel sheets according to our examples had a good surface appearance and high adhesion. In contrast, the comparative examples had poor surface appearance and poor adhesion of the coating.

EXAMPLE 4

Hot-dip galvanized steel sheets were produced from the steel having the composition listed in Table 1, the remainder being Fe and incidental impurities in the same manner as in Example 1, except that the conditions listed in Tables 5 to 7 were employed. Evaluation was performed in the same way as in Example 1 except for adhesion of the coating of unalloyed hot-dip galvanized steel sheets.

Unalloyed hot-dip galvanized steel sheets were subjected to a ball impact test. Adhesion of the coating was evaluated by peeling off a processed portion with a cellophane adhesive tape and by visually inspecting the processed portion for peeling of the coated layer. In the ball impact test, the mass of the ball was 1.8 kg, and the drop height was 100 cm. The diameters of the impact portions were ¾ and ⅜ inches.

-   -   Double circle: Neither peeling of the coated layer for ¾ or ⅜         inches     -   Circle: No peeling of the coated layer for ¾ inches, but slight         peeling of the coated layer for ⅜ inches     -   Cross: Peeling of the coated layer

Tables 5 to 7 show the evaluation results.

TABLE 5 (Example) Electrolytic First heating step treatment Second heating step Dew Heating Holding Charge Treatment Dew Heating Holding H₂ point temperature time density time H₂ point temperature time No Steel (%) (° C.) (° C.) (s) (C/dm²) (s) (%) (° C.) (° C.) (s) 202 A 5.0 −40 800 300 20 1 5.0 −35 800 200 203 A 5.0 −40 800 300 20 3 5.0 −35 800 200 204 A 5.0 −40 800 300 20 40 5.0 −35 800 200 205 B 10.0 −40 860 250 50 1 10.0 −35 800 280 206 B 10.0 −40 860 250 50 5 10.0 −35 800 280 207 B 10.0 −40 860 250 50 30 10.0 −35 800 280 208 F 5.0 −40 850 300 10 1 10.0 −35 800 200 209 F 5.0 −40 850 300 10 5 10.0 −35 800 200 210 F 5.0 −40 850 300 10 35 10.0 −35 800 200 211 H 5.0 −40 800 300 10 1 10.0 −35 860 200 212 H 5.0 −40 800 300 10 10 10.0 −35 860 200 213 H 5.0 −40 800 300 10 20 10.0 −35 860 200 Coating Alloying treatment treatment step step Al Alloying concentration temperature Surface No (%) (° C.) appearance Adhesion Product Remarks 202 0.191 — ◯ ◯ GI Example 203 0.191 — ◯ ⊙ GI Example 204 0.191 — ◯ ⊙ GI Example 205 0.190 — ◯ ◯ GI Example 206 0.190 — ◯ ⊙ GI Example 207 0.190 — ◯ ⊙ GI Example 208 0.190 — X X GI Comparative example 209 0.190 — X X GI Comparative example 210 0.190 — X X GI Comparative example 211 0.189 — ◯ ◯ GI Example 212 0.189 — ◯ ⊙ GI Example 213 0.189 — ◯ ⊙ GI Example

TABLE 6 (Example) Pickling Electrolytic step after First heating step treatment electrolytic Second heating step Dew Heating Holding Charge Treatment treatment Dew Heating Holding H₂ point temperature time density time Weight loss H₂ point temperature time No Steel (%) (° C.) (° C.) (s) (C/dm²) (s) (g/m²) (%) (° C.) (° C.) (s) 214 A 5.0 −40 800 300 20 1 0.34 5.0 −35 800 200 215 A 5.0 −40 800 300 20 3 0.29 5.0 −35 800 200 216 A 5.0 −40 800 300 20 40 0.38 5.0 −35 800 200 217 B 10.0 −40 860 250 50 1 0.33 10.0 −35 800 280 218 B 10.0 −40 860 250 50 5 0.33 10.0 −35 800 280 219 B 10.0 −40 860 250 50 30 0.33 10.0 −35 800 280 220 F 5.0 −40 850 300 10 1 0.12 10.0 −35 800 200 221 F 5.0 −40 850 300 10 5 0.12 10.0 −35 800 200 222 F 5.0 −40 850 300 10 35 0.12 10.0 −35 800 200 223 H 5.0 −40 800 300 10 1 4.8 10.0 −35 860 200 224 H 5.0 −40 800 300 10 10 4.8 10.0 −35 860 200 225 H 5.0 −40 800 300 10 20 4.8 10.0 −35 860 200 Coating Alloying treatment treatment step step Al Alloying concentration temperature Surface No (%) (° C.) appearance Adhesion Product Remarks 214 0.191 — ◯ ◯ GI Example 215 0.191 — ◯ ⊙ GI Example 216 0.191 — ◯ ⊙ GI Example 217 0.190 — ◯ ◯ GI Example 218 0.190 — ◯ ⊙ GI Example 219 0.190 — ◯ ⊙ GI Example 220 0.190 — X X GI Comparative example 221 0.190 — X X GI Comparative example 222 0.190 — X X GI Comparative example 223 0.189 — ◯ ◯ GI Example 224 0.189 — ◯ ⊙ GI Example 225 0.189 — ◯ ⊙ GI Example

TABLE 7 (Example) Pickling step before Electrolytic First heating step electrolytic treatment Second heating step Dew Heating Holding treatment Charge Treatment Dew Heating Holding H₂ point temperature time Weight loss density time H₂ point temperature time No Steel (%) (° C.) (° C.) (s) (g/m²) (C/dm²) (s) (%) (° C.) (° C.) (s) 226 A 5.0 −40 800 300 0.34 20 1 5.0 −35 800 200 227 A 5.0 −40 800 300 0.29 20 3 5.0 −35 800 200 228 A 5.0 −40 800 300 0.38 20 40 5.0 −35 800 200 229 B 10.0 −40 860 250 0.33 50 1 10.0 −35 800 280 230 B 10.0 −40 860 250 0.33 50 5 10.0 −35 800 280 231 B 10.0 −40 860 250 0.33 50 30 10.0 −35 800 280 232 F 5.0 −40 850 300 0.12 10 1 10.0 −35 800 200 233 F 5.0 −40 850 300 0.12 10 5 10.0 −35 800 200 234 F 5.0 −40 850 300 0.12 10 35 10.0 −35 800 200 235 H 5.0 −40 800 300 4.8 10 1 10.0 −35 860 200 236 H 5.0 −40 800 300 4.8 10 10 10.0 −35 860 200 237 H 5.0 −40 800 300 4.8 10 20 10.0 −35 860 200 Coating Alloying treatment treatment step step Al Alloying concentration temperature Surface No (%) (° C.) appearance Adhesion Product Remarks 226 0.191 — ◯ ◯ GI Example 227 0.191 — ◯ ⊙ GI Example 228 0.191 — ◯ ⊙ GI Example 229 0.190 — ◯ ◯ GI Example 230 0.190 — ◯ ⊙ GI Example 231 0.190 — ◯ ⊙ GI Example 232 0.190 — ◯ Δ GI Example 233 0.190 — ◯ ◯ GI Example 234 0.190 — ◯ ◯ GI Example 235 0.189 — ◯ ◯ GI Example 236 0.189 — ◯ ⊙ GI Example 237 0.189 — ◯ ⊙ GI Example

The hot-dip galvanized steel sheets and galvannealed steel sheets according to our examples had a good surface appearance and high adhesion. In contrast, the comparative examples had poor surface appearance and poor adhesion of the coating.

INDUSTRIAL APPLICABILITY

We provide a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet each having high strength, a good surface appearance, and good adhesion of the coating. For example, application of a high-strength hot-dip galvanized steel sheet or high-strength galvannealed steel sheet to automobile structural members can improve mileage due to weight reduction of automotive bodies. 

1.-7. (canceled)
 8. A method of producing a high-strength hot-dip galvanized steel sheet comprising: a first heating step of holding a steel sheet at a temperature of 700° C. to 900° C. or 700° C. to 900° C. for 20 to 600 seconds in an atmosphere having a H₂ concentration of 0.05% to 25.0% by volume and a dew point of −45° C. to 0° C., the steel sheet having a composition of C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 5.00% or less, P: 0.100% or less, S: 0100% or less, and Al: 0.100% or less on a mass percent basis, the remainder being Fe and incidental impurities; an electrolytic treatment step of subjecting the steel sheet after the first heating step to electrolytic treatment in an alkaline aqueous solution at a charge density of 1.0 to 400 C/dm², the steel sheet acting as an anode; a second heating step of holding the steel sheet after the electrolytic treatment step at a temperature of 650° C. to 900° C. or 650° C. to 900° C. for 15 to 300 seconds in an atmosphere having a H₂ concentration of 0.05% to 25.0% by volume and a dew point of 0° C. or less; and a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.
 9. A method of producing a high-strength hot-dip galvanized steel sheet comprising: a first heating step of holding a steel sheet at a temperature of 700° C. to 900° C. or 700° C. to 900° C. for 20 to 600 seconds in an atmosphere having a H₂ concentration of 0.05% to 25.0% by volume and a dew point of −45° C. to 0° C., the steel sheet having a composition of C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 8.00% or less, P: 0.100% or less, S: 0100% or less, and Al: 0.100% or less on a mass percent basis, the remainder being Fe and incidental impurities; an electrolytic treatment step of subjecting the steel sheet after the first heating step to electrolytic treatment in an alkaline aqueous solution at a charge density of 1.0 to 400 C/dm², the steel sheet acting as an anode; a pickling step of pickling the steel sheet after the electrolytic treatment such that a pickling weight loss is 0.05 to 5 g/m² on an Fe basis; a second heating step of holding the steel sheet after the pickling step at a temperature of 650° C. to 900° C. or 650° C. to 900° C. for 15 to 300 seconds in an atmosphere having a H₂ concentration of 0.05% to 25.0% by volume and a dew point of 0° C. or less; and a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.
 10. A method of producing a high-strength hot-dip galvanized steel sheet comprising: a first heating step of holding a steel sheet at a temperature of 700° C. to 900° C. or 700° C. to 900° C. for 20 to 600 seconds in an atmosphere having a H₂ concentration of 0.05% to 25.0% by volume and a dew point of −45° C. to 0° C., the steel sheet having a composition of C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 3.00%, Mn: 8.00% or less, P: 0.100% or less, S: 0100% or less, and Al: 0.100% or less on a mass percent basis, the remainder being Fe and incidental impurities; a pickling step of pickling the steel sheet after the first heating step such that a pickling weight loss is 0.05 to 5 g/m² on an Fe basis; an electrolytic treatment step of subjecting the steel sheet after the pickling step to electrolytic treatment in an alkaline aqueous solution at a charge density of 1.0 to 400 C/dm², the steel sheet acting as an anode; a second heating step of holding the steel sheet after the electrolytic treatment step at a temperature of 650° C. to 900° C. or 650° C. to 900° C. for 15 to 300 seconds in an atmosphere having a H₂ concentration of 0.05% to 25.0% by volume and a dew point of 0° C. or less; and a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.
 11. The method according to claim 8, wherein the electrolytic treatment in the alkaline aqueous solution in the electrolytic treatment step is performed for 2 seconds or more.
 12. The method according to claim 8, wherein the composition further includes at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis.
 13. The method according claim 8, wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.
 14. A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to claim
 8. 15. The method according to claim 9, wherein the electrolytic treatment in the alkaline aqueous solution in the electrolytic treatment step is performed for 2 seconds or more.
 16. The method according to claim 10, wherein the electrolytic treatment in the alkaline aqueous solution in the electrolytic treatment step is performed for 2 seconds or more.
 17. The method according to claim 9, wherein the composition further includes at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis.
 18. The method according to claim 10, wherein the composition further includes at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis.
 19. The method according to claim 11, wherein the composition further includes at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis.
 20. The method according claim 9, wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.
 21. The method according claim 10, wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.
 22. The method according claim 11, wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.
 23. The method according claim 12, wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis.
 24. A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to claim
 9. 25. A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to claim
 10. 26. A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to claim
 11. 27. A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to claim
 12. 